1. Field of the Invention
The present invention relates to aircraft propulsion systems and, more specifically, a cross-flow fan propulsion system.
2. Description of the Prior Art
The cross-flow fan (CFF), developed in 1893 by Mortier, is used extensively in the HVAC industry. The fan is usually long in relation to the diameter, so the flow approximately remains 2-dimensional (2D). The CFF uses an impeller with forward curved blades, placed in a housing consisting of a rear wall and vortex wall. Unlike radial machines, the main flow moves transversely across the impeller, passing the blading twice. FIG. 1 shows a typical heating, ventilation, and air conditioning (HVAC) configuration. For an aircraft installation, the propulsor must ingest and expel the flow in a linear manner to produce forward thrust. The conventional HVAC-type CFF housing, characterized by approximately a 90 degree turn from inlet to outlet, is not well suited for this application
Inherent in all designs is a vortex region near the fan discharge, called an eccentric vortex, and a paddling region directly opposite. These regions are dissipative, and as a result, only a portion of the impeller imparts useable work on the flow. The cross-flow fan, or transverse fan, is thus a 2-stage partial admission machine. The popularity of the cross-flow fan comes from its ability to handle flow distortion and provide high pressure coefficient. Effectively a rectangular fan, the diameter readily scales to fit the available space, and the length is adjustable to meet flow rate requirements for the particular application. Since the flow both enters and exits the impeller radially, the cross-flow fan is well suited for aircraft applications. Due to the 2D nature of the flow, the fan readily integrates into a wing for use in both thrust production and boundary layer control.
In addition to increased propulsive efficiency, embedded propulsion provides reduced noise and increased safety, since the propulsor is now buried within the structure of the aircraft (e.g. no exposed propellers). Also, based on the methods in Ref. 4, by eliminating the engine pylon/nacelle support structure, the aircraft parasite drag can be reduced by up to 18 to 20%, thus improving cruise efficiency and range.
Attempts to provide a cross-flow fan in aircraft wings haven been unsuccessful. For example, some designs uses cross-flow fans embedded within the middle of a conventional airplane wing. Other designs distribute fully embedded cross-flow fans near the trailing edge of a conventional transport aircraft, with shafts and couplings connecting them to wing-tip and root-mounted gas turbines. Air is ducted into the fan from both wing surfaces, and expelled out at the trailing edge. These designs, however, limits the fan size and ducting. Also, the CFF may not be a viable option for high-speed applications due to compressibility effects (i.e. choking). These configurations fall short of expectations due to poor fan placement and poor housing design. These deficiencies result in low fan performance, reduced circulation control, and low thrust production.